The present invention relates to a fuel assembly for a boiling water reactor which is adapted, during operation of the reactor, to allow cooling water to flow upwards through the fuel assembly while absorbing heat from a plurality of fuel rods, whereby part of the cooling water is transformed into steam, and where the fuel assembly comprises a steam pipe through which the steam flows upwards through the fuel assembly towards the outlet end thereof.
In a boiling water nuclear reactor, moderated by light water, fuel exists in the form of fuel rods, each one containing a stack of pellets of a nuclear fuel arranged in a cladding tube. A fuel bundle comprises a plurality of fuel rods arranged in parallel with one another in a certain definite, normally symmetrical, pattern, a so-called lattice, and are retained at the top by a top tie plate and at the bottom by a bottom tie plate. A fuel assembly comprises one or more fuel bundles, each one extending along the main part of the length of the fuel assembly and being surrounded by a substantially square fuel channel.
The core is immersed into water which serves both as coolant and as neutron moderator. The fuel assemblies are arranged vertically in the core and spaced from each other. During operation, the water is admitted through the bottom of the fuel assembly and then flows upwards through the fuel assembly past the fuel rods. The fuel rods emit heat which is taken up by the water which starts boiling, whereby part of the water is transformed into steam. The water and the steam are passed out through the upper end of the fuel assembly.
The produced steam is delivered to turbines which drive generators where electrical energy is generated.
A disadvantage with a boiling reactor is the high proportion of steam in the upper part of the fuel assembly. When the proportion of steam rises in the coolant, its ability to carry off heat from the fuel rods is reduced, thus increasing the risk of dryout, which in turn leads to an increase of the risk of fuel damage. Still another problem with a high steam volume in the fuel is that steam is much inferior to water as moderator, which results in the moderation being insufficient whereby the fuel is utilized inefficiently. In the lower part of the fuel assembly, the moderator consists of water whereas the moderator in the upper part of the fuel assembly consists of both steam and water. This means that the fuel in the upper part of the fuel assembly cannot be utilized efficiently. It is, therefore, desirable to keep down the steam volume in the coolant while at the same time maintaining the steam generation at a high level.
U.S. Pat. No. 5,091,146 discloses a fuel assembly which attempts to achieve a separation of the steam flow and the water flow in the upper part of the fuel assembly by arranging a pipe above one or more part-length fuel rods, that is, fuel rods extending from the bottom tie plate but terminating below and at a distance from the top tie plate. When designing the pipe, there are primarily two problems which have to be solved, namely, how the steam, which is continuously produced along the upper part of the fuel assembly, is to enter the pipe, and also how the water, which is deposited on the inner walls of the pipe, is to be carried away. These problems have been solved by providing the envelope surface of the pipe with a number of openings arranged axially at different levels. Some of the openings are formed so as to carry away the water which is accumulated on the inner surfaces of the pipe, and other openings are formed for passing the steam into the pipe. The openings which are intended for passing in the steam are complicated in their design and it is doubtful whether the steam can really find its way through these openings. An additional disadvantage is that the openings are complicated to manufacture.
The object of the invention is to produce a fuel assembly with a steam pipe, which in an efficient way separates steam from water, which passes the steam out of the fuel assembly, and which is simple to manufacture.
What characterizes a fuel assembly according to the invention will become clear from the appended claims.
A fuel assembly according to the invention comprises a plurality of steam pipes arranged one above the other with their longitudinal axes parallel to the longitudinal axis of the fuel assembly. During operation, the fuel assembly is arranged vertically in the core and steam flows upwards through the steam pipes. The steam pipe has an inlet for the steam in one of its ends, the inlet end, and an outlet for the steam in its other end, the outlet end. The steam pipes are arranged spaced from one another such that openings are formed between the steam pipes. These openings have two functions; for one thing, steam is to flow into the steam pipes and, for another, the water which is formed on the insides of the steam pipes is to be passed out. The envelope surfaces of the steam pipes therefore need not be provided with special openings for admission of steam and discharge of water.
To separate the water from the insides of the steam pipe and prevent it from following the steam flow into the next steam pipe, the outlet and inlet of the steam pipes are designed such that the internal radius of the outlet is larger than the external radius of the inlet. The distance between two steam pipes shall be so large that it provides a sufficient inflow area for the steam while at the same time it must not be so large that the separated water has time to be deflected to such an extent that it follows the steam up into the next steam pipe. Preferably, the opening between the steam pipes shall have an area which is of the order of magnitude of near the cross-section area of the steam pipe.
The water on the inside of the steam pipe forms a coherent water film. In one embodiment of the invention, the outlet end of the steam pipe is provided with means for collecting water from the water film and leading the collected water towards the outlet. In this way, large water drops are formed in localized paths. These large water drops are not deflected as easily as smaller water droplets, which reduces the risk of the water being brought in with the steam. When the water drops are collected in paths, almost water-free paths are also formed between the paths with water drops, and in these water-free paths the steam may flow into the steam pipe without being obstructed. In this way, the risk of the water drops preventing the steam from flowing into the steam pipe is reduced.
To facilitate the inflow of steam to the steam pipe, the oppositely positioned inlet and outlet ends may be designed tapering towards the openings for venturi effect.